This application claims priority under 35 USC 119 of Japanese Patent Application No. 2000-16479 filed on Jan. 26, 2000, the entire disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an apparatus for applying a voltage to a transfer roller for the purpose of transferring a toner image formed on a photosensitive body of an image forming apparatus onto a recording sheet, and especially to an apparatus equipped with a control means that applies a test voltage to the transfer roller and programs (decides and adjusts) the transfer bias. The present invention also relates to an image forming method employing said apparatus.
2. Description of the Related Art
In a conventional electrophotographic image forming apparatus, a toner image is transferred to a recording sheet as follows: an electrostatic latent image is formed on a photosensitive body (drum) by means of an exposing unit, and visualized with a toner supplied from a developer unit. The visualized image is a toner image, which is then transferred to the recording sheet. Finally the recording sheet bearing the toner image is fixed through a fixing unit to be a copy. A discharging member such as a corotron or a contact-transfer member such as a transfer roller may be used in order to transfer the image from the photosensitive body. One of the advantages of using a contact-transfer member such as a transfer roller over the corotron is that there is no ozone generated since unlike the corotron no corona discharge occurs. This results in elimination of such means as an ozone disposal unit in the exhaust system of an image forming apparatus.
The voltage application to said transfer roller as the transfer bias is greatly influenced by the temperature within the apparatus and also in the area surrounding it. At a low temperature, a voltage of a relatively large value should be applied (not larger than 3.5 kV), and the voltage decreases as the temperature within the apparatus increases and is set within the range between 0.5 and 2 kV. To control the transfer voltage, for example, a test voltage is applied to set the bias during the initializing procedure of the image forming apparatus. Later, after a predetermined number of sheets are recorded, a feed back sequence (hereinafter FB sequence) to apply a test voltage again is performed to change the setting of the transfer bias voltage.
However, in a typical image forming apparatus, in many cases a set of documents are placed to make a large set of copies in succession unless the machine allows only one document at a time and requires replacement of the document each time. In addition, many image forming apparatus have a function to set the number of copies at the maximum of 99. Successive printing within that limit is available. In this type of successive printing of large number of sheets, the temperature inside the apparatus rises because of the heat conducted from the fixing unit and the like even when performing a single job.
While a set of jobs are performed, there is a tendency that each time said FB sequence is performed, the values of detected currents keep increasing. So, the voltage applied to the transfer roller becomes gradually smaller. Therefore, even though a test voltage is applied, and the voltage to be applied is decided based on the detected current in the FB sequence, the value of such voltage to be applied tends to be the same due to the saturation. If the decided value after FB sequence always remains the same as the one used before the FB sequence, there is no use performing FB sequences. In addition, if FB sequences are repeatedly conducted during the series of jobs, the time needed for the completion of the series of jobs tends to be longer by the duration of the FB sequences.
It is thus an object of the present invention to solve the problems stated above concerning the FB sequence against the transfer roller.
According to one aspect of the present invention, there is provided an image forming apparatus provided with a transfer roller and performing a sequence during jobs in order to decide a value of the transfer bias by applying a test voltage, characterized in that whenever the saturation is reached by the voltage to be applied, which is decided based on the detected currents obtained when the test voltage is applied, said FB sequence will no longer be performed at least during the job. When the detected current at the time of the application of the test voltage exceeds a certain level, it may be considered that the saturation has been reached. The transfer bias may be determined based on the detected current when the test voltage is applied, and also on the temperature at that moment.
With the image forming apparatus of the present invention, the time required until completion of the job is shortened because wasteful sequences are omitted after it is decided that the saturation has been reached by the transfer voltage that is determined based on the current detected upon application of the test voltage in the FB sequence. Furthermore, by deciding and/or adjusting said transfer bias based on the conditions including both the detected current and temperature in said FB sequence, it is possible to more accurately decide and adjust the transfer bias in view of the environment. Moreover, users have to wait less because the time required for the printing in large number is reduced. Further, the decreased time of operation for each component member of the image forming apparatus means the increased life of the processing units.